The mechanism of excitation-contraction (E-C) coupling in crayfish skeletal muscle more closely resembles that of mammalian cardiac muscle than that of vertebrate skeletal muscle in terms of its dependence on Ca2+-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR). Our preliminary results using crayfish muscle as a working model for CICR have led to the formulation of a new hypothesis of a subdomain within the junctional gap that separates t-tubule and SR to explain how CICR can be tightly controlled with inward Ca2+ currents through the t-tubule membrane while remaining relatively insensitive to Ca2+ released from the SR. The present application seeks: 1) to take advantage of the ease of experimentation with crayfish muscle to study the process of CICR in detail, 2) to perform computer modelling of crayfish E-C coupling as a specific example of local control theory, 3) to compare these findings with results of equivalent experiments on myocytes isolated from rat ventricle. Crayfish muscle experiments will include direct demonstration of intrasarcomeric Ca2+ concentration gradients, determination of the number of Ca2+ ions required to activate each ryanodine receptor, and a test to discriminate between two alternative control models, one first proposed by the P.I. and the other by the coinvestigator. Ca2+ movements in the postulated subdomain as well as in the rest of the junctional gap will be modeled to pinpoint the locations of the two Ca2+ sensitive sites within the gap. Parallel experiments will be carried out on isolated cardiac myocytes together with caged Ca2+ experiments, and studies of I(Ca) inactivation and SR Ca2+ release inactivation to determine how well our crayfish model of CICR may explain CICR in heart muscle and whether it is modified by the presence in heart of ryanodine receptors not intimately associated with dihydropyridine receptors.